Exploration and production of mineral deposits, primarily petroleum and natural gas, but also including, for example, water and various inorganic minerals, often involves the creation of boreholes with rotary cutters. Rock cuttings are continually removed from the borehole during the excavation process. In relatively shallow boreholes (less than a few hundred feet), rock removal is accomplished, for example, by injecting a compressed gas through a hollow drill stem. Rock cuttings are removed through an annulus between the drill stem and the bore hole. In deep boreholes or boreholes in geological formations housing water or other fluids, a liquid is used in like manner to remove rock cuttings. Such liquids are typically referred to as drilling fluids. Additives such as, for example, barite are often added to the drilling fluids to increase density such that the drilling fluid hydrostatic pressure at depth is greater than the hydrostatic pressure of fluids within the geological formation at like depth. The increase in drilling fluid density helps prevent fluids from the geological formations from entering the borehole and inhibits blowouts when high-pressure deposits are encountered during drilling.
Although drilling fluids confer numerous advantages to the drilling process, the drilling fluids are usually at sufficient pressures to invade permeable rock formations. Such permeation is undesirable, since it can interfere with petroleum recovery. Materials are often added to drilling fluids to plug pores in permeable rock formations by forming a filter cake that is impermeable to drilling fluid. For instance, bentonite, a clay, is commonly used for this purpose. Other additives such as, for example, cellulose, polymers, asphalt, GILSONITE® and calcium carbonate are sometimes added to improve filter cake properties. Although bentonite and similar materials reduce rock formation permeability, the filter cakes are not completely impermeable to the drilling fluid, which leads to several problematic situations. For example, penetration of water can cause clay formations to swell, which results in reduced production. Rocks such as, for example, shales can build up a pressure gradient from absorbed drilling fluid such that rock layers can spall off into the drilling fluid. This ‘washout’ can jam drill stems and generate rough surfaces that increase wear. Alternatively, the borehole can become plugged. Furthermore, when drilling fluid penetrates into rock, original fluid in the rock is displaced. When production potential is evaluated using logging tools, a potential production zone may be missed because of petroleum displacement by drilling fluid. Moreover, thick filter cakes can build up with additives currently in use, which increases wear on drill stems and reduces the annulus size for flow of drilling fluids. Another failure mode occurs when additives fail to prevent drilling fluid penetration into highly permeable, large-pore rock formations. Rapid flow of drilling fluids into such rock formations can hydraulically clamp the drill stem to the rock surface, resulting in a condition known as ‘stuck pipe’.
In view of the foregoing, development of improved drilling fluid compositions that prevent or substantially reduce the penetration of drilling fluids into rock formations would be of considerable interest. Such drilling fluid compositions would provide benefits in drilling operations through, for example, reducing formation damage, producing thinner filter cakes, reducing fluid loss into rock formation pores, preserving original rock formation pressure, reducing wear on drilling tools, and reducing the likelihood of drill stem hydraulic adhesion or stuck pipe.